1. Field of the Invention
The present invention relates to a method and an apparatus for producing a toner for electrophotography.
2. Discussion of the Background
Conventionally, image forming apparatuses such as copiers and printers are required to produce higher quality images, and therefore a toner for forming an image has smaller particle diameter and is required to have fluidity and uniform chargeability. Accordingly, various methods of adding, mixing and efficiently adhering various external additives on a surface of a mother toner powder are suggested.
For example, Japanese Laid-Open Patent Publication No. 63-85756 discloses a method of adhering fine particles on a surface of a core material with a mechanical heat energy principally involving an impact force; Japanese Laid-Open Patent Publication No. 63-139366 discloses a method of removing a fine powder which is not adhered after mixing and adhering the fine powder; Japanese Laid-Open Patent Publication No. 10-10781 discloses a method of firmly adhering a surface treatment agent upon application of an instant heat; and Japanese Laid-Open Patent Publication No. 10-95855 discloses a method of uniformly adhering a surface treatment agent on a mother toner particle with a spheric mixer rotating two blades at a high speed.
Namely, in order to firmly adhere a surface treatment agent on a surface of a mother toner particle, a method of strengthening an impact force while softening the surface of the mother toner particle upon application of heat or a method of aggressively heating a resin surface of the mother toner particle without using the impact force is available. In addition, particles which are not adhered tend to be present in these methods.
Further, recent toners for electrophotographies have lower temperature fixability, and binder resins having a low glass transition temperature are preferably used in the toners. In addition, materials having a low melting point is included in toners for electrophotographies in many cases such that the toners have releasability from image forming apparatuses.
When mother toner particles including a material having a low melting point and a binder resin having a low glass transition temperature are stirred and mixed with an external additive to adhere the external additive to the mother toner particles, the material having a low melting point begins to melt and the resultant toner properties deteriorate unless the mother toner particles are stirred and mixed at a temperature at which the toner or the material having a low melting point melts. In addition, the particles agglutinate each other due to anastomoses and have to newly be classified or removed, which is not efficient in producing a toner.
Japanese Laid-Open Patent Publication No. 2000-267354 discloses a method of adhering inorganic fine particles on a surface of a mother toner particle by stirring and mixing at comparatively a low temperature based on a standard temperature of a glass transition temperature of a binder resin included in the mother toner particle. Specifically, a Henschel mixer using a vertical and cylindrical tank stirs and mixes.
When a surface treatment agent is adhered to a mother toner particle as a fluidity auxiliary agent for the purpose of improving fluidity of the resultant toner, the mother toner particle needs to have concavity and convexity to firmly receive the surface treatment agent. Therefore, the surface treatment agent needs to be mixed and stirred with the mother toner particle so as to collide therewith by a proper impact force. Accordingly, a Henschel mixer or a high speed mixer having a vertical and cylindrical tank and a rotating blade rotating at comparatively a high peripheral speed is conventionally and preferably used. In the method disclosed in Japanese Laid-Open Patent Publication No. 2000-267354, such mixers are also used.
However, particularly when a surface treatment agent controlling a charge is used, not only the surface treatment agent is adhered to a mother toner particle but also a part or a whole of the surface treatment agent has to be uniformly buried on the surface of the mother toner particle to be firmly fixed thereon. A presence of a toner on which a surface treatment agent controlling a charge is insufficiently fixed impairs uniform friction charge of toners and causes an image stain called background development.
Therefore, the surface treatment agent and the mother toner particle have to be mixed at such a stirring speed as gives a sufficient impact force therebetween to fix the surface treatment agent on the mother toner particle.
Particularly when a surface of a mother toner particle including a material having a low melting point and a binder resin having a low glass transition temperature is treated with a surface treatment agent to aim at a charge control, the mother toner particle and surface treatment agent have to be mixed at a low temperature at which the material having a low melting point does not melt and given sufficient impact force therebetween such that the surface treatment agent is fixed on the surface of the mother toner particle.
Because a mixer such as a Henschel mixer and a high speed mixer usually has a flat bottom and a cylindrical wall, as FIG. 1 shows, an air turbulence is generated in the mixer when stirring at a high speed and a mother toner powder irregularly moves. Therefore, the mixer has a drawback that the powder not only stagnates on the bottom thereof but also tends to adhere on the cylindrical wall.
Further, it is difficult to sufficiently fix the surface treatment agent on the mother toner particle in the Henschel mixer or high speed mixer because a maximum peripheral speed of a rotating blade thereof is 40 m/sec in practical use.
A hybridizer for use in an impact method in a high speed air stream is known as a stirring mixer capable of rotating its blade at a higher speed than that of the Henschel mixer or high speed mixer. The stirring mixer can sufficiently fix the surface treatment agent on the mother toner particle, but does not have a sufficient cooling mechanism because of being originally used to mix two or more kinds of particles with a heat. Therefore, the stirring mixer cannot practically be used because an amount of the surface treatment agent has to be extremely small when a surface of a mother toner particle including a material having a low melting point and a binder resin having a low glass transition temperature is treated with the surface treatment agent at a low temperature at which the material having a low melting point does not melt.
Thus, conventional mixers cannot sufficiently stir and mix the surface treatment agent and mother toner particle particularly when a surface of the mother toner particle including a material having a low melting point and a binder resin having a low glass transition temperature is treated with the surface treatment agent at a low temperature at which the material having a low melting point does not melt. Surface treatment conditions of individual mother toner particle have unevenness and a mixture of the wholly surface-treated mother toner particle and partially surface-treated mother toner particle causes a charged amount unevenness of individual mother toner particle. Therefore, the resultant image has a stain called background development and an amount of a toner which is not used for forming images and collected increases.
In addition, a surface treatment agent is easily released from a toner when the surface treatment agent is not firmly adhered thereto, and damages a photoreceptor or deteriorates performance of a developer due to a carrier spent.
Methods of preparing a mother toner particle including at least a thermoplastic resin, a colorant, a release agent and optionally various additives include a pulverization method of kneading the thermoplastic resin, colorant, release agent and optionally various additives upon application of heat; pulverizing and classifying the kneaded mixture; and optionally repeating the pulverization and classification, and a polymerization method of polymerizing the thermoplastic resin, colorant and a charge controlling agent by dispersing them as a grease spot in a solvent, etc.
It is indispensable to uniformly fix a charge controlling agent on a toner powder prepared by the polymerization method to prepare a highly reliable toner because it is difficult to take the charge controlling agent in the grease spot.
However, it is quite significant to make the charge controlling agent present only close to the surface of the mother toner particle in terms of cost reduction because the charge controlling agent is expensive.
Because of these reasons, a need exists for a method of producing a toner in which a charge controlling agent is used as a surface treatment agent, mainly present closely to the surface of the mother toner particle and fully fulfills its function.